Scan Time; Overview - Omron SYSMAC C Series User Manual

Appendix C
Scan time
Overview
up ERROR indicator
Sets error flag and lights
ERROR indicator lit?
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When a programmable controller operates, that is, when it executes its
program to control an external system, a series of operations are
performed inside the PC. These internal operations can be broadly
classified into four categories:
Common processes such as resetting the watchdog timer and checking
the user program memory
Data input and output
Instruction execution
Peripheral device command servicing
Scan time
The total time required for a programmable controller to perform all of
these internal operations is called scan time. The following flowchart and
diagram illustrate these internal operations.
Power application
Clears I/O and internal
relays; resets all timers
Checks 1/0 unit
connection
Resets watchdog
timer
=. —— =}
Checks hardware and
user program memory
Checking OKed?
YES
Inputs data from input
unit to I/O relays and
outputs data from I/O.
relays to output unit
Initial processing on power application
Common processes
Resets watchdog timer
and program counters
Executes instructions
in the user program
Program
execution ended?
Scan time of PC
Instruction execution
~~, YES
Resets watchdog timer
Services commands
from peripheral devices
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